Effect of black residue on the mechanical properties of Sn-58Bi epoxy solder joints

The effect of black residue (BR) formation on the mechanical properties of Sn-58Bi epoxy solder surface finished with an electroless nickel-immersion gold (ENIG) were evaluated with various reflow process conditions. The BR was formed around the solder ball after reflow process, and their area were variated with ramp time and peak temperatures. As peak temperature increased from 160 to 190 degrees C, the area ratio of the BR decreased from 50.3 to 18.0%. According to XRD, FT-IR, and XPS analyses, we assumed the most probable composition of the BR was epoxy or flux residue with Sn and Bi as well as carboxylate salts such as Bi(RCOO)(3) and Sn(RCOO)(2). These were generated by the carboxyl-based flux to prevent oxidation and reduce the surface tension of the molten solder in the epoxy-based Sn-58Bi soldering. The shear strength increased by 31.3% as the peak temperatures increased from 160 degrees C to 190 degrees C, while the fracture energy increased by 18.3%. We also investigated the effects of the addition of epoxy on the mechanical properties of Sn-58Bi solder joints under various reflow conditions. The bonding characteristic of the Sn-58Bi epoxy solder joints were elucidated via chemical analysis and by observing the fractures.